1. Field of the Invention
The present invention relates to a micro-hardness measurement method for measuring the hardness of a test sample by forming an indentation or impression by pushing an indenter into a test sample, and a micro-hardness meter therefor, and in particular, to a micro-hardness measurement method and a micro-hardness meter, which are best suitable for calculating the hardness of a test sample in which a hard surface layer is formed on the surface of a soft substrate.
2. Description of the Prior Art
Conventionally, as a measurement method of the hardness, a diagonal length measurement system by which the hardness is obtained based on the length of a diagonal line of an indentation formed by an indenter, such as Vickers hardness, and a depression depth measurement system by which the hardness is obtained based on the depth of a depression, exist. In the diagonal length measurement system, the hardness is obtained by using a calculation expression of Hardness=(Maximum load)/(Surface area of permanent depression) from the surface area of a permanent depression calculated based on the diagonal length of an indentation. Also, in the depression depth measurement system, the hardness is obtained by calculating the surface area of a depression based on a push-in depth of an indenter when an indentation is formed and by using the above-described calculation expression.
Here, in order to measure the diagonal length of an indentation by a microscope in the diagonal length measurement system, it is necessary to form an indentation whose diagonal length is at least 10 micrometers or so. However, where the hardness of a test sample in which a hard film is formed on the surface of a substrate, there was a problem that the hardness of only the film cannot be obtained since influences on the hardness of the substrate are exerted or an indenter penetrates the film. In addition, since the depression depth of the indentation is made equivalent to the thickness of the film and the indentation is influenced by the substrate, there was another problem that it is difficult to read the diagonal length of the indentation by using an optical type microscope.
On the other hand, since, in the depression depth measurement system, the depression depth of an indentation is measured based on the push-in amount of an indenter, it is possible to measure the hardness of an object even if the object is a micro indentation for which the diagonal length cannot optically be measured.
However, since, in the depression depth measurement system, a test sample is deflected if an indenter is forcedly pushed therein where the elastic deformation ratio of the test sample is great when forming an indentation, the push-in depth of the indenter is made greater than the depth of the indentation (permanent depression), wherein the surface area of a depression calculated from the push-in depth of the indenter becomes greater than the surface area of the indentation (permanent depression) after the load is unloaded, and there is still another problem that the hardness calculated by the (maximum load)/(surface area of permanent depression) is underestimated.
Hereinafter, a description is given of the problem with reference to the drawings.
FIG. 6 is a schematic diagram depicting a state where an indenter is pushed in a test sample in which a hard surface layer is formed on the surface of a soft substrate.
In the drawing, reference numeral 100 denotes a test sample, 101 denotes a soft substrate such as a synthetic resin, etc., 102 denotes a hard surface layer such as glass, which is formed on the surface of the substrate 101. Reference numeral 103 denotes an indenter formed to be like a pyramid.
When the indenter 103 is pushed in the surface layer 102 of the test sample 100 with a load given to the indenter 103, the substrate 101 and the surface layer 102 are elastically deformed as depicted in FIG. 6(b). If the indenter 103 is further pushed in, the distortion is increased, wherein the elastic deformation of the substrate 101 and the surface layer 102 is also increased, and at the same time, plastic deformation occurs (FIG. 6(c)). When the load given to the indenter 103 is released and is unloaded, if the indenter 103 has not reached the substrate 101, the deflection of the surface layer 102 and the elastic deformation of the substrate 101 are eliminated, wherein the plastic deformation appears on the surface of the test sample 100 as a permanent depression (FIG. 6(d)).
Therefore, the push-in depth of the indenter 103 becomes deeper than the depth of an indentation (permanent depression) after the load is unloaded, and the surface area of the depression calculated from the push-in depth of the indenter 103 becomes wider than the surface area of the indentation (permanent depression) after the load is unloaded, wherein the hardness calculated by the (maximum load)/(surface area of the permanent depression) is underestimated.
As prior art to solve the problem, for example, Japanese Patent No. 3510411 discloses a micro-hardness measurement method including the steps of: measuring depths of a plurality of indentations or impressions formed by different loads, respectively; calculating respective depth correction values in which respective depth measurement values are corrected in response to the profiles of the tip ends of the indenters; measuring respective lengths of diagonal lines of the plurality of indentations or impressions; inversely calculating the depths of the indentations based on the hardness calculated on the basis of the respective measurement values of the diagonal lines; correcting the actually measured values of the depths of the indentations by means of a correction equation defined on the basis of the correction values of depth and the inversely calculated values of the depth; and calculating the hardness based on the after-correction depth and the load for forming indentations.
Japanese Published Examined Patent Application No. Hei-5-20691 discloses an ultra-micro-hardness meter including means for calculating a proportional constant K and a dynamic hardness of a test sample by the least squares method based on a predetermined calculation equation from a load P given to an indenter at an optional measurement point, a displacement d of the indenter at the load, and a constant cc determined by the profile of the indenter.
Japanese Published Unexamined Patent Application No. 2001-349815 discloses a method for determining the hardness of a thin film, including the steps of: obtaining the ratio of hardness to elastic modulus of the thin film of a measurement object based on an approximate expression in which the relationship between work done by an indenter when the indenter is loaded for being pushed in a thin film of the measurement object, work done by the thin film onto the indenter when being unloaded, and ratio of hardness to elastic modulus of various materials are linearly approximated; next obtaining the contact area of the indenter at the maximum load; and acquiring the hardness of the thin film.
However, the following problems in the prior art described above exist.
In the art disclosed by Japanese Patent No. 3510411, since the correction equations differ from each other depending on the profile and material of the indenter, the depth of an indentation and the length of the diagonal line are measured whenever the profile and material of the indenter change, wherein it is necessary to calculate the correction value and carry out correction. Therefore, there is a problem that the operation is very cumbersome.
In the art disclosed by Japanese Published Examined Patent Application No. He-5-20691, an error occurs when calculating the proportional constant K and the dynamic hardness of a test sample by means of the least squares method, and there is another problem that the measurement accuracy is lowered. Additionally, since the hardness is calculated with the profile of the indenter taken into consideration based on the relationship between the load P and displacement d at an optional measurement point, no consideration is paid to the push-in depth of the indenter, wherein there is a premise that the hardness is constant in the depth direction of the test sample. Therefore, where the hardness is calculated in regard to a test sample the hardness of which changes in the depth direction as in a test sample in which a hard surface layer is formed on the surface of a soft substrate, there is still another problem that the error becomes remarkably large, depending on the push-in depth of the indenter.
In the art disclosed by Japanese Published Unexamined Patent Application No. 2001-349815, since an error occurs when the relationship of the ratio of hardness to elastic modulus of various materials is linearly approximated, and the error is accumulated when calculating the hardness, there is still another problem that the measurement accuracy is lowered. Also, since the hardness H is obtained by a calculation equation of H=Pmax/A from the maximum load Pmax and the contact area A of an indenter at the maximum load, the art is a hardness measurement method aiming at metal materials the test sample of which is unlikely to be deflected and elastically deformed, and there is a problem that the method is short of applications.